The present invention relates to a wireless coordinate reading system using an electromagnetic coupling phenomenon. More specifically, the invention relates to a coordinate reading system which can detect operation states of numerous switches provided on a coordinate indicator.
Various methods have been proposed for detecting operation states of switches of a coordinate indicator in the conventional wireless coordinate reading system. One known method is to transfer a switch signal by using infrared rays. Another known method is to modify a resonance frequency of a resonant circuit and make the resonant circuit resonate with an excitation signal, and then detect the operation states from a degree of the phase shift.
Further, as to a method for detecting a position of a coordinate indicator, a cord can be eliminated from the coordinate indicator because of the existence of the invention (hereinafter referred to as Prior Application No. 1) which was proposed by the applicant of the present invention before the present application. Brief description will be given for this invention. Two groups of sense lines are arranged along each of xy rectangular coordinate axes respectively, and each line of the one group of sense lines is sequentially excited by an excitation circuit. When a coordinate indicator having a resonant circuit which resonates with the above excitation signal is brought close to the sense lines, induced signals which are induced on the other group of sense lines are sequentially processed by a signal processing circuit. Then, a position of the coordinate indicator, that is, a coordinate is detected from the level of the induced signals which are processed.
The method using infrared radiation to transfer the switch signal has directivity, and therefore has the following drawbacks. The structure becomes complicated and the system becomes heavier and larger, so that the operability deteriorates. For instance, the coordinate indicator itself requires a transmitter and a large capacity cell. Further, there is another drawback that the difference in phase data between respective switches is too small to detect operation states of numerous switches only from the degree of a shift in phase, and therefore stable detection is difficult.